Vibrio aestuarianusClade A and Clade B isolates are associated with Pacific oyster (Crassostrea gigas) disease outbreaks across Ireland

Abstract

Bacteria from the Vibrionaceae family have been implicated in mass mortalities of farmed Pacific oysters (Crassostrea gigas) in multiple countries, leading to substantial impairment for growth in the sector. In Ireland there has been concern thatVibriohave been involved in serious summer outbreaks. There is evidence thatVibrio aestuarianusis increasingly becoming the main pathogen of concern for the Pacific Oyster industry in Ireland. While bacteria belonging to theVibrio splendidusclade are also detected frequently in mortality episodes, their role in the outbreaks of summer mortality are not well understood. To identify and characterise strains involved in these outbreaks, 43Vibrioisolates were recovered from Pacific oyster summer mass mortality episodes in Ireland from 2008-2015 and these were whole genome sequenced. Among these, 25 were found to beV. aestuarianus(implicated in disease) and 18V. splendidus sensu lato(role in disease undetermined). Two distinct clades ofV. aestuarianus— Clade A and Clade B — were found that had previously been described as circulating within French oyster culture. The high degree of similarity between the Irish and FrenchV. aestuarianusisolates points to translocation of the pathogen between Europe's two major oyster producing countries, probably via trade in spat and other age classes.V. splendidusisolates were more diverse, but the data reveal a single clone of this species that has spread across oyster farms in Ireland. This underscores thatVibriocould be transmitted readily across oyster farms. The presence ofV. aestuarianusClades A and B in not only France but also Ireland adds weight to the growing concern that this pathogen is spreading and impacting Pacific oyster production within Europe.